I downloaded a video from youtube.
I place it on my kindle fire (using the USB (sold separately)
then when I went to my videos and tapped on library, nothing is displayed.
The extension of the file was a .mp4
I'd also like to know more video file extensions Kindle Fire supports.

Thanks alot!
How about if the video I downloaded are .avi and its green lantern.
How will I convert that to .mp4 or other formats?

I have used RealPlayer to convert video for many years. Handbrake is also pretty good. RealPlayer can convert video to many different formats. Click on Tools and RealPlayer Converter. Drag whatever file you want converted into the conversion box, choose the conversion format, and click Start. RealPlayer takes care of the rest.

Not possible to use some app to play other type of content? For instance, the player which came with my smartphone used an mp3 codec which was buggy (the internal codec). I downloaded an app that uses the ffmpeg codec, which plays pretty much anything under the sun.

Not possible to use some app to play other type of content? For instance, the player which came with my smartphone used an mp3 codec which was buggy (the internal codec). I downloaded an app that uses the ffmpeg codec, which plays pretty much anything under the sun.

Who's gonna convert all videos to mp4 just to play it on Fire?

Sure. I just sideloaded MoboPlayer 1.2.176. It plays a lot of different formats including Quicktime. It uses the FFmpeg decoder. You can download from the Moboplayer website.

If you do a lot of YOUTUBE downloading you can use an app called YOUTUBE Free and it download right into the fire. You simply do a search of the clip you want, hold down on picture and you get a list of things you can do like view or download. Great for getting movie clips and music videos.

I just downloaded Moboplayer today so I haven't had a lot of experience with it yet, but it seems really good. I like the easy controls for adjusting the backlight and volume without having to pause. Most of my videos are in Quicktime format and relatively short as I like to create time lapse movies from series of photos taken with my Canon dSLR. I did read a review that said Moboplayer doesn't remember where you left off on a paused movie if you close the app, so that would be problematic if you are watching a 2 hour movie. But maybe they can fix that before long.